An advantage of DSA equipment is that it can be used for the evaluation of cardiovascular diseases. Doctors and clinicians have been attempting to use DSA equipment to aid in making quantitive measurements of stenosis (narrowing of the blood vessels) due to deposits on the vessel walls or, in a wider sense, any narrowing of the blood vessels. However, until now such measurements have not proven to be reliable. See for example, an article by Kruger R A, Anderson R E, Koshler R, Nelson J A, Sorenson J A, and Morgan T, entitled: "The Non-Invasive Evaluation of Cardiovasucular Dynamics Using a Radiographic Device", in Vol. 139, Radiology, pp 301 et seq. (1981). Another article of interest on the same subject is entitled: "Assessment of Quantatitive Indices of Arterial Stenosis Derived from Intravenous Digital Subtraction Angiography" by Peck W W, Slutsky R A, Brahme F, and Higgins C B. The article appeared in the American Heart Journal of Sept. 1984, pp 591 et seq.
The parameter which is usually used to quantify stenosis is the area ratio: EQU S=(An-As)/An
where An is the cross-sectional area at a "normal" location, and As is the cross-sectional area at a stenotic location.
Stenosis, as noted above, usually results from deposits (mostly fatty sometimes referred to as "scales") on the walls of the blood vessel. These deposits cause a narrowing of the aperture through which the blood flows, reducing the blood flow into the organ which is fed by the blood vessel. However, these deposits are very irregular: they occur in random locations along the vessel and where they do occur they appear in different angular positions. Thus, the cross-section of the blood vessel, which is circular when healthy without deposits on the walls (The circular shape is due to the combined action of the natural elasticity of the wall tissue and the pressure exerted by the blood.) becomes irregular.
In the subtracted image, numbers are assigned to pixels according to the amount of contrast medium (radiopaque dye) along the line of sight from the X-ray source to the portion of the image intensifier corresponding to the pixel. Those numbers are referred to for short and for historical reasons as "densities". For example, within the blood vessels the pixels have a high density value and outside the blood vessel the pixels have low density values. Thus, where the blood vessel passes over a portion of the pixel the assigned density number will be somewhere between the highest density level and the lowest density level. The pixels with no portion of a blood vessel thereon have the lowest density values. The pixels completely covered by a blood vessel have the highest density values. In actual fact, the amounts of contrast medium along the line of sight are proportional to the product of the opacity density of the contrast medium and the distance traversed by the X-ray line of sight within the contrast medium. However, the term "density" may be misleading as the opacity density may be assumed to be constant and where that assumption holds the amount of contrast medium is really proportional to the above distance. This distance is usually perpendicular to the plane of the image and therefore provides information about the third (unseen) dimension. The thicker the blood vessel the higher are the density values of the pixels covered by those blood vessels.
Existing methods for quantifying stenosis can be categorized as: geometric and densitometric. The geometric methods rely on measurements of the size of the blood vessel passageways in the X-ray image plane only. The densitometric methods rely in addition on the "density" data measurements (actually the thickness) of the blood vessel passageways and background in the X-ray images. Any measurement using only the location (x,y) of selected pixels are termed geometric, while, measurements using the additional density information in the pixels are termed densitometric.
All known geometric methods suffer from this irregularity of cross-section of the blood vessel described above as they use a small number of views (usually only one) of the vessel and assume a regular shape (circular for single views, elliptic for two views). Therefore, for example, if the cross-section of a stenotic blood vessel has an elongated shape then when the stenotic location is viewed so that the short side of the remaining passageway or opening of the blood vessel is towards the observer the geometric method will underestimate the cross-sectional area (sometimes grossly) and thus overestimate the stenosis. Conversely, if the long side is towards the observer the geometric method will tend to overestimate the cross-sectional area and sometimes overlook the stenosis completely.
Densitometric methods usually suffer from digitization errors and overlying (underlying) background. That is, there are too few pixels (pixels = picture elements, derived from the digitization of the DSA image) across the blood vessel for a good fit to a mathematical function, and the background density (caused by underlying and overlying vessels which are below the resolution threshold, by scatter from other tissue etc;) makes direct integration inaccurate. In both cases quantum noise affects the results both by randomly modifying the values themselves and by randomly dithering the blood vessel edges, making it difficult to ascertain where to start measuring, integrating or fitting.
All presently existing methods for quantifying stenosis are deemed unusable by the clinicians, surgeons and diagnosticians. Repeating the measurement by two observers, and even by the same obeserver at a later time, gives widely differing quantitative results.
Although there are some who claim it is more important to measure the effect of the stenosis on the blood flow (the "significance" of the stenosis), it is generally considered important to be able to give a quantitative measure of the stenosis which is independent of the observer and relatively insensitive to the accuracy of the manual part of the operation.
A completely different problem is the definition of the above mentioned "normal" cross-section area. What is really needed is the cross-sectional area of the blood vessel without stenosis. This is an idealized quantity that can not be measured, and some approximation has to be chosen.
It may seem that an "atlas" of blood vessels may be compiled, giving the "normal" cross-section areas at each point (or at selected points). However, normal human variability combined with normal variations in imaging practice (size of image intensifier, distance of X-ray source and/or detector) make this impracticable. The scaling factor between the body and the image depends on the exact depth of the vessel within the body and only numbers proportional to the cross-sectional area can be derived.
Most existing methods use as an approximation to the "normal" cross-sectional area a measurement of the cross-sectional area at a nearby portion of the blood vessel which is considered by the physician to be "normal". This method suffers from drawbacks such as:
A. The selected portion may nevertheless be afflicted. It may be slightly stenotic, by an amount not discerned by the physician but which may affect the quantization of the stenosis. It may also be aneurismic (distended), due to an increase in blood pressure behind the impediment to free flow, also by an amount undiscerned by the physician but affecting the result. B. The normal shape of the blood vessels is tapering, starting with a very large diameter at the exit from the heart (the aortal) or at the entrance to it (the two vena cavae) and becoming progressively narrower as the distance from the heart increases. This narrowing is not at a constant rate but rather steplike or ramplike. Some stretches of vessel have a nearly constant diameter while others have relatively rapidly changing diameters. At the latter streches, using a different location to measure the "normal" cross-sectional area of the stenatic portion introduces an additional error.
Thus there is an ever present need for quantifying stenosis. More particularly there is a need for finding a method of quantifying stenosis that is relatively observer independent and capable of using measurements made by relatively unskilled technicians. The method should preferrably include a method of obtaining reliable and repeatable approximations of "normal" cross-sectional areas.